FIG. 1 is a diagram illustrating the radio frame structure of a TDD system. In the diagram, a 10 ms radio frame includes two half-frames of the same length, and the length of each half-frame is 5 ms. Each half-frame further includes 7 normal time slots (also called subframes) and 3 special time slots which are a DwPTS, a GP and an UpPTS. The total length of the 3 special time slots DwPTS, GP and UpPTS is 275 us.
Wherein the length of the DwPTS time slot is approximately 83.333 us, a primary synchronization (P-SCH) signal is transmitted in the middle bandwidth of the time slot, and is mainly used for downlink synchronization. No data is transmitted in other bandwidth of the time slot;
The length of the GP time slot is 50 us, and the time slot is used for the protection of switching from a downlink time slot to an uplink time slot;
The length of the UpPTS time slot is approximately 141.67 us, an uplink random access signal is transmitted in the time slot and is mainly used in downlink synchronization.
Among the 7 normal time slots, the length of each time slot is 675 us, in order to simplify the system implementation, TS0 is always specified as a downlink time slot, and a secondary synchronization (S-SCH) signal used for downlink synchronization is transmitted in the middle bandwidth of the last symbol of the time slot. TS1 is always specified as an uplink time slot, and other 5 normal time slots, namely TS2˜TS6, may be specified as an uplink time slot or a downlink time slot flexibly according to the service demand.
When a certain normal time slot is a downlink time slot, each time slot includes 8 or 9 symbols being transmitted by way of Orthogonal Frequency Division Multiplexing (OFDM) (the number of the symbols is related to cyclic prefixes of the symbols, when the cyclic prefix is a normal cyclic prefix, each time slot includes 9 symbols, and when the cyclic prefix is an extended cyclic prefix, each time slot includes 8 symbols). In the time slot, downlink control channel is usually located in the first several symbols thereof, and downlink shared channel for transmitting normal user data is located in the last several symbols of the time slot.
When a certain normal time slot is an uplink time slot, each time slot includes 8 or 9 symbols which are transmitted by way of single-carrier. (the number of the symbols is related to cyclic prefixes of the symbols, when the cyclic prefix is a normal cyclic prefix, each time slot includes 9 symbols, and when the cyclic prefix is an extended cyclic prefix, each time slot includes 8 symbols). In the time slot, uplink control channel is usually located on both sides of the system bandwidth, and the duration is one time slot; while uplink shared channel for transmitting user normal data is located in the remaining position of the system bandwidth, and the duration is also one time slot.
The frame structure of the TDD system is similar with that of the current TD-SCDMA system. When the TDD system coexists with the current TD-SCDMA system at adjacent frequencies, if only the proportion of switching between uplink time slots and downlink time slots being set by the TDD system is the same as that of the TD-SCDMA system, then the mutual interference between the TDD system and the TD-SCDMA system may be avoided effectively.
However, there are several problems in the above-mentioned TDD system as listed bellow:
The setting of the GP is very inflexible. In this system, the length of the GP time slot is 50 us, when the system is required to support wider coverage, the length of the GP must be extended. Methods for extending the GP can be to reserve the UpPTS time slot as the GP time slot, or to reserve the UpPTS time slot and the TS1 as the GP time slot (wherein part of TS1 symbols can not be reserved as the GP because an uplink control channel is usually located on two sides of the system bandwidth, and the duration is one time slot; if part of TS1 symbols are reserved as the GP, the reception of the uplink control channel will be greatly degraded, nor should part of the ISO symbols be reserved as the GP, because the P-SCH/S-SCH signals are transmitted in the last two symbols of TS0, if part of the TS0 symbols are reserved as the GP, the user may be unable to receive the P/S-SCH signals, which are the signals a user has to receive first before accessing the system, and if a user cannot receive the P/S-SCH signals correctly, nor can the user access the system).
The P/S-SCH signals are located at the border of the switch from the downlink time slot to the uplink time slot, as illustrated above, the P/S-SCH signals are the signals a user has to receive first before accessing the system, therefore, the power of the P/S-SCH signals are usually higher than other signals, in a cellular environment, because of the high power of the P/S-SCH signals, the reception of an uplink time slot signal will be severely impacted.
The utilization efficiency of the DwPTS time slot is not high. When the system bandwidth is wide, since there is only the P-SCH signal transmitted in the middle bandwidth (1.25 MHz) of the system bandwidth, the utilization efficiency of the DwPTS time slot is very low.
Therefore, it is necessary to provide a new method for transmitting signals in a TDD system, by which not only the signals transmitted can coexist with an existing TD-SCDMA system at adjacent frequencies, but also the above-mentioned problems can be solved.